The present invention relates to radiocommunications with mobiles, and more particularly the methods of controlling access by mobile terminals to resources of a radiocommunication network.
Many radiocommunication systems use methods of controlling transmit power in order to reduce the level of interference between the various communications. This power control has a particular importance in spread spectrum systems using Code Division Multiple Access (CDMA). In these systems, several terminals can share the same frequency at every moment, the separation of the channels on the radio interface resulting from the quasi-orthogonality of the spread codes respectively applied to the signals sent over those channels. In other terms, for a given channel, the contributions of the other channels are seen as noise.
In particular, on the uplink, transmit power control limits the transmit power of the mobiles close to a base station to prevent the signals that they send masking the signals originating from more distant mobiles. In general, the power control methods use loop power control: the base station takes measurements on the signal received from a mobile (power, signal-to-interferer ratio (C/I), etc.), and transmits commands to increase or reduce power on the downlink in order to tend towards a given quality objective. These methods cannot be used before a radio link is established between the base station and the mobile. In particular, they do not allow idle mobiles to determine the level of power at which they must send any random access requests.
For UMTS (“Universal Mobile Telecommunications System”) systems, the transmit loop power controls on the uplink are described in technical specification 3G TS 25.401, version 3.3.0, published in June 2000 by the 3GPP (“3rd Generation Partnership Project”), pages 20-21.
For the power of the first signals sent by a mobile terminal to a base station, particularly for a new communication, these loop power controls are not operational, because the base station has not received the previous signal from the mobile terminal allowing it to take the required measurements. The mobile terminal then estimates the power of these first signals according to another procedure based on the attenuation of the signals sent by the base station and received by the mobile terminal. The base station broadcasts a beacon signal indicating the power at which it has sent it. The receipt of these beacon signals allows the idle mobile to determine the resources used by the base station with which the link is the best (cell selection) and to evaluate the attenuation of the signal from that station. From this it deduces an initial power for transmission of the radio signals to the selected base station, the power equalling the degree of attenuation.
In certain circumstances, particularly when the mobile terminal is very close to the receive antenna of the base station, the result of this estimate may be a very low transmit power. Such is the case for example of a call from a maintenance agent working on the base station itself and using his radio terminal.
Now a radio terminal, due to its construction, has a minimal radio transmit power below which it is not capable of transmitting. Technical specification 3G TS 25.101, version 3.6.0, published in March 2001 by the 3GPP, recommends a minimal transmit power by UMTS mobile terminals of −50 dBm (section 6.4.3, page 13).
If the transmit power estimated for the random access request is below this minimal power, the mobile terminal sends the random access request with its minimal transmit power (see technical specification 3G TS 25.214, version 5.4.0, published by the 3GPP in March 2003, section 6.1).
If this transmit power is clearly greater than the power estimated from the attenuation measurements, this transmission risks generating significant noise for the other radio signals received by the base station and therefore damaging the quality of transmission of the communications in progress to which these other signals belong.
To limit this effect, WO 99/65158 proposes that, when a mobile terminal is too close to a base station, this base station transmits a “first command” to the said mobile terminal, making it enter a degraded operation mode, in order to prevent that terminal harming the communications of other mobile terminals. This “first command” may in particular be generated after an access request and be transmitted instead of the channel allocation. The effect of this command may be to inhibit or delay the establishment of a link between the mobile terminal and the base station in question.
This solution inhibits the procedure of establishing a communication. This inhibition is performed systematically when the mobile terminal is considered too close to the base station, without consideration of the type of service envisaged.
In particular, if the mobile terminal requests a specific communication service while being close to a base station, for example if it attempts to make an emergency call, this call risks being impossible due to it being inhibited during the establishment procedure. Now, for obvious reasons, it is desirable that certain communications such as emergency calls can be made in all circumstances.
WO 02/098017 proposes to inhibit the transmission of network access signals by a mobile terminal when the difference between its minimal transmit power and the estimated initial transmit power exceeds a predefined threshold, that is to say when the mobile terminal is too close to a base station of the access network. This manner of proceeding can be used to deal easily with the problem of emergency calls since the terminal can itself override the inhibition of the access signals when it knows that it is in the process of requesting an emergency call. But since this solution is not standardized, it will in practice be applied only by a small proportion of the population of terminals in circulation. Now the inhibition of network access requests from a terminal too close to a base station essentially benefits the other terminals situated in the cell, which suffer less interference. This observation is of the type that restrains recourse to this type of precaution, despite its value for the network user community.
An object of the present invention is to restrict these disadvantages in particular by avoiding systematically inhibiting all call attempts without distinction for a mobile terminal too close to a base station of an access network, and to do this without counting on the mobile terminal itself.